The chemical or electric properties of an electrolyte solution such as electric conductivity and sparking voltage become main factors which determine the properties of the electrolytic capacitors.
A solution of an amine salt of maleic acid or phthalic acid in an aprotic organic polar solvent such as N,N-dimethylformamide or gamma-butyrolactone is used for low voltage electrolytic capacitors which have good low temperature characteristics.
However, as the impedance of electrolytic capacitors has become lower, electrolyte solutions having higher electric conductivity have been desired.
A solution of boric acid or ammonium borate in an ethylene glycol solvent has been used as an electrolyte solution for medium to high voltages because it has a high sparking voltage. As is well known, this electrolyte solution contains large amounts of water formed during esterification of ethylene glycol and boric acid, and large amounts of water vapor formed at temperature exceeding 100.degree. C. causes distortion in the appearance of the armoring case and deteriorates the electric properties of the electrolytic capacitors. For this reason, it is impossible to use this electrolyte solution at temperature over 100.degree. C.
To overcome this disadvantage, it was proposed to use an ammonium salt of a branched-chain aliphatic dicarboxylic acid where the water content becomes extremely small [Japanese Laid-Open Patent Application No. 45014/81 and Japanese Laid-Open Patent Application No. 27013/82 (corresponding to U.S. Pat. 4,469,610)].
However, the above electrolyte solutions using a solvent composed mainly of ethylene glycol have a poor electric conductivity at low temperature.
It may be possible to use an aprotic solvent such as N,N-dimethylformamide or gamma-butyrolactone having a broader freezing point-boiling point range than ethylene glycol together with the aforesaid ammonium salt of a branched-chain aliphatic dicarboxylic acid in order to improve the low temperature characteristics, but very little of these ammonium salts solve into such solvents.
Japanese Laid-Open Patent Application No. 78522/84 (corresponding to U.S. Pat. No. 4,473,864) discloses an electrolyte solution having high electric conductivity composed of a solution of a quaternary ammonium salt of a dibasic carboxylic acid HOOC--CH.sub.2).sub.n COOH (4.ltoreq.n.ltoreq.8) in amide solvents with 2 to 10% by weight of water. This electrolyte solution, however, is not fully satisfactory in electric conductivity, sparking voltage, and temperature range.